Supported chromium catalysts have long been a dominant factor in the production of high density olefin polymers such as polyethylene. As originally commercialized, these catalysts were used in solution polymerization processes. However, it early became evident that a more economical route to many commercial grades of olefin polymers was a slurry process, that is, a polymerization process carried out at a temperature low enough that the resulting polymer is largely insoluble in the diluent. However, certain control techniques which are easily carried out in solution polymerization systems become more difficult in the slurry system. This is particularly true with regard to control of molecular weight. In a solution system, the temperature can simply be increased in order to provide a lower molecular weight, higher melt flow polymer. However, in slurry systems, there is a practical limit on temperature increases, since the point is quickly reached wherein the polymer goes into solution and thus the value of the slurry system is lost.
In order to allow maximum utilization of the more commercially desirable slurry system, modifications of the catalyst were developed so as to allow the production of higher melt flow polymer. One such modification which has become commercially successful is the formation of a cogel of silica and titania. This cogel is aged and water removed by means of azeotropic distillation or washing with a water miscible organic compound to produce the support for the chromium. Such catalysts have demonstrated a remarkable ability for producing high melt flow polymer. Since chromium must be incorporated at some point in the catalyst preparation, it has been suggested to co-gel the chromium with the silica or if titanium is desired, to form a tergel by gelling silica, titania and chromium together.
Unfortunately, it has been found that co-gellation of the chromium with the silica results in a catalyst which does not produce as high a melt flow polymer as would otherwise be produced.